The invention relates to antenna arrays of the type in which the radiating elements are excited by proximity coupling to a source of electromagnetic surface waves developed on an adjacent conductive structure.
While the invention broadly concerns antenna arrays having the foregoing characteristics, a preferred form of the invention, as disclosed herein, is deployed as a space borne, earth directed beam antenna for transmitting solar-to-electrical converted power earthward for collection by ground based receiving antennas.
It has previously been proposed that solar power can be efficiently converted to electrical power at space borne satellites and then transmitted to earth in the form of microwave electromagnetic energy, which as used herein refers to the higher frequencies of the radio band, such as from about 300 megahertz to 4,000 megahertz. By collecting solar energy in large surface area arrays of solar cells located in orbit above earth's atmosphere, a significantly greater degree of the available solar energy can be captured and transformed into electrical power than is possible using earth based solar cells. After the solar-to-electrical power conversion however, it becomes necessary to provide means for efficiently transmitting the electrical power to earth. For this purpose, it has been proposed that the satellite structures that support the arrays of solar cells be adapted to mount high frequency transmitting equipment for transforming the solar-derived electrical power into high frequency electromagnetic energy and for beaming such energy earthward to ground based receiving antennas. At frequencies suitable for transmitting relatively large amounts of power through the atmosphere, antenna structures in the microwave band are considered most effective.
One proposed transmit antenna structure for this purpose comprises rafts of juxtaposed microwave guides, each of rectangular cross section and slotted along the larger dimension face of each wave guide, in a known manner, for collectively radiating a highly directional beam of radiated electromagnetic energy. By creating large planar arrays of such rafted wave guides, a relatively efficient transmit antenna can be created. While such an antenna configuration is feasible, there are certain disadvantages inherent in its structure which would be desirable to overcome or avoid. First, the slotted wave guide raft configuration when assembled in space, results in a relatively heavy, bulky structure. Additionally, the nature of the component wave guides are such that the structure is highly susceptible to overheating due to the combined efforts of resistive losses in the structure during the transmission of power, and due to the interception of solar energy as a result of the closed face configuration inherent in its architecture.
Furthermore, there has not been a satisfactory proposal for arranging the components in a high density package for being launched into satellite orbit for assembly in space. The antenna structures that must be used, when assembled in space, are of a flattened configuration spanning a relatively large area, and are arranged to face the earth, so as to achieve the cumulative power transmitting capability necessary for beaming useable amounts of electrical power to earth. Because of this generally planar configuration of the antenna structure, it is not practical to launch an assembled antenna into orbit; rather, the constituent elements for the antenna arrays must be consolidated in high density pay load packages and launched in this form into space and assembled by crews while in a stable orbit. This requirement places a severe constraint on the nature and shape of the ultimate structure, and more especially on the components that are used to assemble the final structure. Also, the weight-to-power capability of the antenna arrays becomes an important factor and desirably this ratio is held to a minimum so that each launched payload package has that much greater value in terms of transmitting power versus cost capacity.
Preliminary studies show that a six-foot by six-foot array of rafted, slotted wave guides may have an earth weight in the neighborhood of 120 to 160 lbs. For overall efficiency of the satellite power station, it has been estimated that the total weight for an antenna module having a comparable level of power transmitting capability, should be around 20 lbs., far less than the projected weight of the previously proposed antenna structure.
In addition to the above factors, a practical antenna array for this purpose must have a high degree of rigidity in order to obtain phase stability needed for power transmission efficiency. All elements of an antenna array must have a predetermined, constant phase relationship in order to produce a predictable and stable set of propagating wave fronts which constructively combine to form the directional characteristics of a beam. If excessive expansion, contraction, twisting, or other deformation of the antenna structure occurs in reaction to the environmental conditions in space, such as the above-mentioned solar pressure, then certain sections of the array will exhibit phase shifts with respect to other elements, thereby reducing the overall beam-forming capability of the array.
Accordingly, it is one object of the invention to provide an antenna array suitable for use as the transmit antenna in a solar driven satellite power station, in which the electrical and structural characteristics of the array overcome one or more of the foregoing disadvantages or difficulties associated with previously proposed configurations.
Another object of the invention is to provide an antenna array for such a space power station that combines electrical and mechanical properties to yield a novel antenna structure characterized by a high degree of transmit efficiency and beam forming phase stability when arrayed in a generally planar structure that extends over the large area needed to transmit useful levels of electrical power earthward.
Still a further object of the invention is to provide a novel antenna structure for microwave frequencies which is characterized by being relatively light in weight, having an open lattice structure that is substantially transparent to solar heat, and being sufficiently rigid so as to achieve a high degree of phase stability.
An additional object of the invention is to provide a modular antenna structure for use in a space power satellite station of the type described above, in which the antenna is characterized by: a high-transmit efficiency such that when a plurality of similar modules are arrayed over a large planar area, the resulting structure is capable of transmitting sufficient electromagnetic energy to be useful for beaming electrical power earthward; a high degree of phase stability to form a concentrated beam of transmitted electromagnetic energy; a low weight-to-power handling ratio; an open framework "see-through" configuration for withstanding solar radiation pressure incident on space borne structures; and an architecture composed of interconnectible elements which prior to assembly have a suitably form factor for being packaged in a high density pay load for being launched into orbit by available propulsion means.